Source Identi�cation and Variation in the Chemical Composition of Rainwater in Southern-Eastern Region of Bangladesh

The present study portrayed a better understanding of the chemical characteristics of rainwater in SouthEastern region of Bangladesh (e.g. Chattogram) as well as to identify the potential sources of different precipitation constituents in the study region that were often unexplored and not well understood. Rainwater pH, major ions, and trace metals were measured in samples collected from �ve different locations with different land use patterns of Chattogram Metropolitan area (CMA) during the two rainy seasons. The samples were tested following standard protocols. The results of this study exhibit variability in rainwater quality across the sites signifying site-speci�c in�uences. The mean concentration of all measured physicochemical parameters, ions and trace metals in rainwater samples were found signi�cantly lower compared to the drinking water quality standard of Bangladesh. In context of ionic constituents, the higher concentrations of nitrate (NO 3- ) and sulphate (SO 42- ) were generally in commercial area. The correlation matrix and principal component analysis (PCA) indicated that NO 3- and SO 4 2- were from anthropogenic sources, e.g. automobile exhaust, incomplete fuel combustions, and industrial emissions. The average concentration of trace metals in rainwater was followed a decreasing order: Zn (cid:0) Cu (cid:0) Fe (cid:0) Cr (cid:0) Mn (cid:0) Pb (cid:0) Cd. Trace metals concentration, especially copper (Cu) and zinc (Zn) were found maximum in the industrial catchment area. The resulting outcomes of this study could be useful to investigate the in�uences of industrial, urban, and agricultural emissions that elaborate the physical processes regulating the chemical characteristic of the atmosphere in the investigated area.


Introduction
Rainwater is considered blessings and is found generally safe and common alternative sources of water around the world that got huge attention of its culture in relation to quality uses in particularly developing countries. Although, rainwater is safe, the chemical compositions at the point of collection is seen altered in uenced by local environment and atmospheric condition; hence, different places might see different elemental concentrations in place that is often key to de ne suitability of uses. Rainwater chemistry is formed through a dynamic interaction between the atmospheric (e.g. cloud) dynamics and the microphysical actions in together with a sequence of 'rainout and washout atmospheric chemical reactions' [1][2][3][4]. Rainwater chemistry signi es the chemical characteristics of the atmosphere through which it falls, helps to understand and investigate the soluble elements exists in rainwater as noted by Wang and Han [5]. Rainwater compositions provide insight into the relative contributions of different atmospheric pollutants, and their regional dispersion [6][7]. Additionally, it also helps to investigate the site-speci c characteristics, such as in uences of emissions from industrial, urban, and agricultural sectors, and the biogeochemical factors that elaborate the physical processes regulating the chemical characteristic of the atmosphere in regional areas and natural biogeochemical cycle [8][9].
Chattogram, a commercial capital and one of the largest port cities in Bangladesh is situated in the eastern coastal zone of Bangladesh [10]. The city is a home of nearly 4.5 million people and the sources of water are in mixed modes of surface and groundwater. However, ground water near the coastal belt see substantially high salinity in dry period due to little or no rainfall along with arsenic in few cases in shallow tube wells at household scale [11][12]. Therefore, people mostly depend on the water supplied by Chattogram Water Supply and Sewerage Authority (CWASA) both for domestic and drinking purposes that are intermittent and fall short in coverage ful lling around 69% demand for city dwellers. In absence of wastewater treatment facilities and poor solid waste management system, the river water quality near city area is found unsuitable, hence, CWASA has to go further upstream covering the great distance for searching comparatively less polluted surface water suitable for treatment for its connected [13][14].
Taking poor quality surface water and groundwater with high salinity and arsenic in consideration, the rainwater culture is an only option left and is seen as blessing due to the volume of precipitation received during monsoon in Bangladesh.
While the potential rainwater harvesting methods and opportunities across Bangladesh are increasingly being studied [15][16][17], the chemical characteristics of rainwater and the variation of relative contributions of different atmospheric pollutants with respect to different land uses remain generally unexplored considering its merit for suitability of uses. There have been surprisingly very few studies devoted to investigating the chemical composition of rainwater around the globe. In Bangladesh particularly in Southern-Eastern region of the country e.g. Chattogram, no such study is found at present. These knowledge gaps are contributed to this study, which describes the results of a comprehensive experimental investigation on rainwater chemistry in Chattogram region across various social and economic zones of the city, e.g. industrial, residential, and urban area. The main purpose of this study to obtain a better understanding of the chemical composition of rainwater across varying geographical locations of Chattogram as well as to identify the potential sources of different precipitation constituents in the study region.

Materials And Methods
The rainwater chemistry was identi ed by comprehensive eld and laboratory tests on various water quality parameters, e.g. pH, turbidity, alkalinity, hardness, major ions, trace metals, etc. The resulting chemical composition of rainwater obtained in this study are then compared with the existing standards of water quality parameters that are applicable to the study region.

Study Area
The city Chattogram is a noteworthy coastal city and commercial hub located in the south-eastern sides of Bangladesh. Chattogram Metropolitan area (CMA) is four times greater than the Chattogram City Corporation (CCC) area, situated as shown in Fig. 1 (22°6′41″N-22°33′38″N, 90°41′E-92°2′E). The city is bounded by the Bay of Bengal on the west, by Halda River on the north-east and, by Karnaphuli River on the south-east. The busiest seaport of Bangladesh is located at the estuary of the Karnafuli River [18].
Due to this port facility, the city is business friendly that's why more than hundreds of oldest and largest industries of Bangladesh established here [19]. The CMA is one of the critical coastal regions of the country with around 4.5 million inhabitants (density 6000/km 2 ) with a growth rate of 2.06% per annum [20].
Geographically, the core city area is plain land and low height hillocks mainly towards the north with a peak elevation of 351 m in Sitakunda [21]. The tropical monsoon climate mostly dominates the weather characteristics of the city. According to the available data from Chattogram station of Bangladesh Meteorological Department (BMD), 2400 mm and 3000 mm annual rainfall occurs in Chattogram region of which 80% of total occurs between May and September (Fig. 2). In which, the mean annual relative humidity is 73.7% and mean monthly relative humidity varies from 58% in January to 86% in August [22]. During dry weather usually mild cold wind blows from the north to the south of the country with very little precipitation. The average maximum and minimum air temperature are 32.3 °C and 13 °C, in summer and winter, respectively [22]. In this study, ve locations are selected addressing different land uses for collecting rainwater samples in which three of them are in the city centre ( Fig. 1), while the remaining two sites are outsides of city centre, one of which is an industrial area named Kumira and another one is a sub-urban residential area named Pahartali. Generally, in a eld, there exist residential, commercial and industrial characteristics, that's why in this study, all of the city based on their land use for speci c purposes are selected, e.g. Figure 1. Rainwater parameters of GEC and Agrabad locations particularly belong to commercial land use might have been in uenced by the road tra c environment as of high tra c volume.
Meanwhile, the highest tra c-induced noise pollution was recorded in the commercial areas (e.g. GEC intersection and Agrabad circle in Fig. 1.) [18,23]. The growing concern for city dwellers is air pollution, mostly caused by automobile emission as the number of vehicles are increasing day by day, in 2010 registered vehicles was 84,391. Still, besides that, there are several thousands of non-registered vehicles [24]. Suspended Particulate Matter (SPM), sulphur dioxide (SO 2 ) and nitrogen oxides (NOx) are the three major components of air pollution in Chattogram city with SPM being the most concentrated [18,25]. Pahartali site is mainly residential in nature where most of the Bangladesh Railway residence is situated.
Kumira is an industrial area located in the north of the CMA one side of which is the Bay of Bengal and other side is hills. Besides, various cement, glass and steel manufacturing industries, South Asian one of the famous ships breaking industry situated here. The soil, water and also air around the ship breaking industries is greatly polluted by toxic substances [26-28]. On the other hand, Chittagong University of Engineering & Technology (CUET) is situated in Raozan Upazila very different from other sites and can be considered pristine site with little or no signi cant pollution, which is not part of CMA. The area is suburban with relatively clean environment where agricultural land use persists on top of academic activities.

Sample Collection
The samples of rainwater were taken on an event basis. Total ninety-ve wet-only rainwater samples were collected from two rainy seasons (June 2018 to October 2018 and June 2019 to October 2019). Rainwater collector shown in Fig. 3 was placed on the roof of building at each of the rainwater sampling sites described above with varying heights ranged 10 to 20 m. First ush tank was incorporated to sperate the rainwater after rst ush by using ballcock. In this study, the water from rainwater tank was used to analyse. In rainwater tank, there also a ballcock used to prevent passing of remaining rainwater stayed in catchment after lling up rainwater tank. It's because, during the time between rainfall and collection there is a chance of contamination by birds and insects since the catchment was open whereas rainwater tank was closed. In this study, rst ush water was not considered for testing since in most of the study it was suggested to let the rst ush water go as it is considered polluted [29][30][31]. Between two rain events, the rainwater tank was cleaned manually while the dry deposition of pollutants primarily in uenced by atmospheric fall out cannot be avoided. Immediately after each rainfall events the rainwater samples were collected by using 500 ml polypropylene bottle. Before collecting, the bottles were washed by using distilled water to avoid cross contamination. Table 1 summarizes the analytical parameters and methods used for determining rainwater quality. Potential of hydrogen (pH), electrical conductivity (EC) and TDS were measured at in situ by using Smart Digital pH meter and HQ40D Multi-meter. The pH meter has been calibrated using 4.0, 7.0, and 10.0 pH buffer solutions. All the samples were stored at 4 °C until analysis. Prior to analysis, samples were ltered by using No. 1 Whatman® lter paper to remove the insoluble particles. The standard analytical protocol following quality control and quality assurance has been maintained for the experiments conducted in the Environmental Engineering Laboratory at CUET.

Physicochemical analysis
All the physicochemical parameters investigated in this study were found well below than the Bangladesh standard in all selected land use locations. The descriptive statistics of physicochemical parameters tabulated in Table 2, while for ease of understanding the average concentrations of the parameters are kept in bar chart plot in. The average pH values ranged from 6.07 to 6.75 in all tested samples which is in slightly acidic range. The EC and TDS were recorded higher in industrial area, indicating presence of more ions dissolved in the rainwater at that location (Fig. 4). Turbidity were found in similar range in all locations slightly higher in commercial area due to the presence of high quantity of dust particles in air of that region, although turbidity is not inevitably a health threat, it may possibly create a health hazard if the suspended particles have adsorbed toxic organic or inorganic compounds [30]. Average concentration of alkalinity and hardness were found somewhat higher in commercial area.

pH value and ionic composition
Natural rainwater is normally considered to be weakly acidic with a pH value of 5.6 when the atmosphere is free from pollution [32][33]. Statistical variation of pH at different land use locations is shown in Fig. 5.
The maximum pH (8.66) was recorded in residential (sub-urban) area whereas minimum value (4.68) was found in industrial area. Considering the variability and anthropogenic input at different sites, the residential catchment in Pahartali relatively clean with no surrounding input sources, exhibit the wide range of pH compared to other sites with urban, commercial and industrial surroundings. The relatively smaller variation in other rial catchment in comparison to sub urban catchment illustrate site-speci c characteristics in uenced by the anthropogenic input (road tra c and industrial emission) during precipitation followed by dry deposition between rain events. The similar variations are also seen in studies elsewhere [34][35].
Descriptive statistics for major ions in rainwater of different locations are given in Table 3. For the all four sampling locations, SO 4 2− was the highest dominant major ion, with an average concentration of 192.5, 274.7, 90.3 and 89.6 µeq/l, for industrial, commercial, residential (urban) and residential (sub-urban) area, respectively. The maximum concentration of NO 3 − and SO 4 2− were found in commercial area where rainwater quality might be in uenced by vehicular emission than other locations. Most of the time vehicles are in tra c congestion in that location which increases vehicular emission [24]. Contribution of NO 3 − and SO 4 2− in commercial area are 36% and 41%, respectively, among all locations as depict in

Trace Metals
A statistical summary of trace metal concentrations in rainwater samples of different selected sites is presented in Table 3. It is observed that the average concentrations of all selected trace metals are well below than the Bangladesh drinking water standard. Elevated concentration of Zn was found in all analyzed samples, although the limiting value of Zn also much higher than other metals. Comparatively, the higher concentration of Zn and Cu was found in industrial area. The mean concentration of copper (Cu) in industrial area was 74.7%, 48.2% and 35.6% greater than the commercial, residential (urban) and residential (sub-urban) area, respectively. Industrial areas are usually in uenced by metal intensive activities, and the widespread use of metals, such as Fe, Cu and Zn in industries, are re ected in higher concentrations in rainwater of these metals [37][38]. The average concentration of Fe was recorded 47.2 µg/l, 36.7 µg/l, 23.3 µg/l and 69.6 µg/l in industrial, commercial, residential (urban) and residential (sub-urban) area, respectively. Pb was only found in the rainwater of commercial area. Highest average concentration (32.3 µg/l) of Cr found in commercial area while lowest average concentration (6.5 µg/l) recorded in residential (sub-urban) area. Mn was found in elevated concentration in commercial area while Cd was found below detection limit in all sampling sites. Atmospheric deposition is one of the signi cant pathways of trace metals presence in urban rainwater and is mostly impacted by site-speci c emissions [37]. As vehicular emissions in the commercial area are ample than those of residential area, this pathway (atmospheric deposition) could contribute to the elevated concentration of trace metals in the commercial site [39].
It is observed that the average concentration of trace metals in all analyzed samples followed a decreasing order of Zn Cu Fe Cr Mn Pb Cd. The percentages of total concentration for selected trace metals in different land use locations are shown in Fig. 7. Cu and Zn, together accounted 80% of total concentration in industrial area while 67% for commercial area, 84% for residential (urban) area and 74% for residential (sub-urban) area.

Worldwide comparison
The precipitation analysis of this study is compared with the data from other areas of Bangladesh as well as worldwide. In Table 4, the physicochemical characteristics of rainwater measured for this study area have been compared with those observed worldwide in other studies. The pH of rainwater samples was in the range from 4.68 to 7.82, with a mean value of 6.63 ± 0.62, i.e. is in the acidic range, reveals the similar characteristics as reported in other parts of the world. The concentrations of TDS and EC were found lower within this work when compared to those observed in other areas, but smaller than Korea. The measured values of alkalinity and hardness for the tested area were higher than those reported in other parts of the world, see Table 4. The concentration of turbidity was much lower than those observed in worldwide but slightly higher than those in another city of Bangladesh, i.e. Sylhet. In general, it is seen that the differences and variability among different sites within city, country, region and globe are re ected in the results, as expected, that signify the in-depth investigation prior to rainwater harvesting potential at sites. The mean concentrations of ions and trace metals in the study area for all samples are presented in Table 5 in conjunction with those reported for other regions around the world. Overall, the concentration of nitrate (NO 3 − ) was found much lower in the investigated area in comparison to those observed in other sampling sites in the world, as shown in Table 5. The sulphate concentration (161.8 µeq/l) was observed to be slightly high, when compared with those reported in the literature for similar sampling locations around the world, except for the Taiwan in the republic of China (238.2 µeq/l). The observed values of NH 4 + were much lower in precipitation reported here compared to those observed in the sampling sites around the world. The PO 4 3− concentration was slightly lower than that of India.   The trace metal concentrations reported here (Fe, Cu, Zn, Pb, Mn, Cr, and Cd) were overall somewhat higher than those reported worldwide, see Table 5. It is evident from Table 5 that the concentration of Zn (203.7 µg/l) was found to be one of the highest concentrations of trace metals in the study area, which was also considerably higher than those found in Mexico, India, and Japan, except for the Jordan (210 µg/l). The Fe concentration (45.1 µg/l) was much lower in the investigated area when compared with those reported in Jordan, and India, but higher than China and Korea. The concentration of Cu (48.5 µg/l) within this study was moderately higher than that reported in Mexico, India, and Japan, but lower than that of Jordan. The mean Pb value was found close to those observed in other sampling sites in Asia, i.e. India and China, but much smaller than those in Jordan and Mexico. Overall, the concentrations of Mn and Cd were reported lower in this work compared to the data worldwide. The Cr value was found somewhat higher than those reported around the world, except for Mexico.

Enrichment factor of trace metals
Enrichment factors (EFs) are generally practiced to nd the source of ions and metals in rainwater [47][48]. Crustal enrichment factor (EF crust ) was used in this study to identify the non-crustal or anthropogenic sources of trace metals in rainwater. EF crust was calculated by using the following equation i.e. Eq. 1 [49]: Where, in Eq. crustal material. In this study iron (Fe) is taken as a reference metals because "iron is the most abundant element, by mass, in the earth, constituting about 80% of the inner and outer cores of earth" [50]. The enrichment factors were calculated by using the composition of continental crustal elements from Rudnick and Gao [51]. In general, an element with an enrichment factor (EF) value signi cantly greater or drastically lower than 1 is assumed to be enriched or diluted relative to the reference source [52][53]. Average enrichment factors for selected trace metals in rainwater are presented in Table 6 for all locations. EF of different metals were found more or less like this study by Uygur et al. [54] and Báez et al. [43]; while they have used Al and Mg as a reference metal. Every metal shows too much higher enrichment factor (especially Cu and Zn) which suggesting the presence of anthropogenic sources. Enrichment of Zn in rainwater like this study has been reported by others, also [44,46]. Mn had much smaller EF values compared to the remainder of the metals, that were also observed greater than one, exhibiting 16 at commercial area, 7.3 at industrial area, 6.8 at residential (urban) area and 3.6 at residential (sub-urban) area, respectively.

Correlation matrix
The correlation matrix is a helpful way to characterize the relationship among the species present in rainwater samples. To investigate the relationship within the rainwater quality parameters, Spearman's rank correlation analysis was done and shown in Fig. 8 (Table 7) showed three PCs with eigenvalues greater than 1 explaining about 69% of the data variability. The parameters which were correlated signi cantly in correlation analysis also shows strong loading in PCA analysis. The three components were rotated using Varimax rotation procedure. A three-dimensional plot for all the variables in the component1, component2, and component3 is illustrated by Fig. 9. The rst component (PC1) described 36% of the total variance and revealed strong loading among TDS (0.943), conductivity (0.930) and hardness (0.819), also moderate loading between pH (0.503) and turbidity (0.730). This clustering of variables points to a common origin and these are in rainwater likely related to environmental conditions.

Conclusions
Detailed measurements have been undertaken to investigate the rainwater quality across different locations in the South-eastern region of Bangladesh. Based on this study, the following conclusions were made - The mean concentration of all tested physicochemical parameters, ions and trace metals in rainwater was found signi cantly lower compared to the drinking water quality standard of Bangladesh.
The concentration of nitrate (NO 3 − ) was found much lower in the investigated area in comparison to those observed in other sampling sites in the world. For the sampled locations, the mean concentration of nitrate (NO 3 − ) and sulphate (SO 4 2− ) was recorded highest in the commercial area.
The observed values of NH 4 + within this study were much lower in precipitation compared to those observed in the sampling sites around the world.
The trace metal concentrations (Fe, Cu, Zn, Pb, Mn, Cr, and Cd) of our study area were overall somewhat higher than those reported worldwide. Higher enrichment factor of metals suggesting the presence of anthropogenic sources. The mean concentration of trace metals in rainwater was followed a decreasing order: Zn Cu Fe Cr Mn Pb Cd in almost every sampling location.
Trace metals concentration, especially copper (Cu) and zinc (Zn) were found maximum in the industrial area. The mean concentration of copper (Cu) in industrial area was 74.7%, 48.2% and 35.6% greater than the commercial area, residential (urban) and residential (sub-urban) area, respectively.
Principal component analysis was performed to nd possible sources of the major species in rainwater. The parameters which were correlated signi cantly in correlation analysis also shows reliable loading in PCA analysis. The results showed that incomplete fuel combustions, industrial emissions, agricultural activities, and also environmental conditions were the dominant sources of the chemical composition in rainwater of Chattogram region.
Prior to this experimental investigations, limited knowledge was available in the chemical composition of rainwater in the southern-eastern region of Bangladesh, which has been attributed in this paper.
Consequently, the data and ndings of this extensive laboratory study would be useful to the engineers and researchers working in the sustainable use of water resources in Bangladesh, but particularly in southern-eastern region of the country.

Declarations
Availability of data and materials Data will be available from the corresponding author on request.
Competing interests